Flash-based webapp Aviary Phoenix is the most impressive image editing tool I've ever seen running in a browser. Free to try, you can do some amazing things with the features on offer, from photo compositing to retouching—the video demonstration embedded above and plenty of tutorials should give you an idea. While not an Adobe Photoshop replacement, if you're stuck with a machine that doesn't have anything better than MS Paint installed, it can be a lifesaver. For beginners, it's a great introduction to the kind of skills and tools available in almost any modern image editor, but with no download necessary. The founders created image editing contest site Worth1000, so you know the tools Aviary is developing will live up to pretty high standards.